The invention relates to a process for the powder metallurgical production of work pieces or tools containing high melting point carbides and/or carbonitrides that are homogeneously distributed in the matrix and PM parts produced according to this process.
In the process for the production of work pieces or tools, particularly when they are produced from alloys containing a high amount of carbon, for example, cold work steels, high-speed steels, and the like, and/or containing high nitrogen concentrations, powder metallurgical processes can be employed to advantage. In the process a molten alloy is atomized to form a powder, this powder is filled into capsules, and a PM (powder metallurgy) part is produced by means of sintering, HIP-ing (hot isostatic pressing), and/or hot-forming and the like. When the particle formed by atomization of a homogeneous melt of the alloy is rapidly cooled, the reaction of the carbon and/or nitrogen with the elements contained in the alloy which elements form carbides and/or nitrides occurs in a brief period of time. As a result, the washing of coarse carbides and/or carbonitrides is prevented from forming during hardening and a uniform distribution of fine particles of these compounds in the powder granules is achieved. The end products, PM parts consequently have a homogeneous distribution of carbides and/or carbonitrides of small granular size in a matrix, which particularly improves the toughness and performance properties.
The usable contents of carbon and nitrogen in the alloy are limited in combination with the amount of carbide-forming and/or nitride -forming elements of the IVa and Va groups, or secondary groups, of the periodic table, because when the amounts of carbon and nitrogen are high, the carbides and/or carbonitrides of the elements already form in the melt due to the high affinities between these elements and carbon and/or nitrogen. These primarily precipitated compounds have high melting points and grow in size in the melt to be mostly block-like and/or dendritic granules, which cannot be reduced even in the atomizing process. This may result in inhomogeneities and scarring in coarse carbides in the resulting PM part, which negatively affects the operating properties of the latter, particularly its toughness qualities.
Attempts have been made in the case of higher concentrations, particularly of the elements C and Nb, to prevent the formation of coarse primary carbide precipitates by the means of technical alloying procedures or influencing the nuclear condition of the melt. However, they have not been able to achieve any significant improvements.
Also proposed in the case of alloys containing elements of more than 3.0 percent in weight which form carbides of the type of MeC and Me.sub.4 C.sub.3, (where Me means metal and C means carbon, or carbides) was superheating at the temperatures far above the usual melting temperatures, for example 1750.degree. to 1800.degree. C., in order to thereby dissolve primary carbide precipitates or to avoid them, and rapid cooling of the alloy from this temperature. The disadvantage here is that the fireproof linings of the furnace for melt and atomization aggregates wear away quickly. Furthermore, at high temperatures the affinities of the elements, for example of niobium and titanium for oxygen, are considerably increased, whereby oxide formations are increased, which causes impurities in the melt and an uncontrollable combustion of the elements.